A cemented carbide used widely as cutting tools for cutting metal, siding members and wear resistant members includes, for example, a WC—Co alloy in which a hard phase composed mainly of tungsten carbide (WC) particles is bonded through a binder phase composed mainly of cobalt (Co), and a WC—Co alloy in which a hard phase called as a β phase (B-1 type solid solution phase) composed of β particles (B-1 type solid solution) composed of carbide, nitride and carbonitride of metals of groups 4, 5 and 6 of the Periodic Table is dispersed. These cemented carbides are utilized as a material for cutting tool which is used to cut general steels such as carbon steel, alloy steel and stainless steel.
In a predetermined depth zone extending from the surface of a cemented carbide from the inside, a binder-phase-riched layer including a high content of Co as a binder phase component exists. It is disclosed that, when a hard coating is formed on the surface of the cemented carbide by forming the binder-phase-riched layer on the entire surface of the cemented carbide, fracture resistance of the cemented carbide is improved (see, for example, patent literature 1).
However, in the cemented carbide disclosed in patent literature 1, although fracture resistance is improved when coated with the hard coating, the hard coating may sometimes peel off, and sufficient adhesion between the cemented carbide substrate and the hard coating may not be achieved. Also, when no hard coating is formed, hardness of the entire surface of the cemented carbide decreases and large plastic deformation occurs on the surface, and therefore cutting resistance increases and the temperature of a cutting edge increases, thus causing a problem that a binder phase existing in the cutting edge gradually reacts with a work material, namely, low welding resistance. In a cemented carbide composed of fine particles in which WC particles in the cemented carbide has a particle size of 1 μm or less, thermal conductivity tends to decrease to cause a problem such as welding. As a result, because of the work material welded to the cutting edge, chipping and sudden fractures are likely to occur, and thus a further improvement in welding resistance on the surface of an alloy has been required.
Patent literature 2 describes that, in a titanium-based cermet made of a nitrogen-containing sintered hard alloy, when the entire surface of the cermet includes a high content of a binder phase of Co or nickel (Ni), or a multi-layered structure exudation layer including a high content of tungsten carbide (WC) is formed, thermal conductivity on the surface of the cermet is improved and thus it is possible to suppress thermal cracking caused by difference between the temperature of the surface increased as a result of cutting and a low temperature inside.
However, even if an exudation layer is formed on the entire surface of a cermet as disclosed in patent literature 2, hardness of the entire surface decreases and large plastic deformation occurs on the surface, and therefore cutting resistance increases and the temperature of a cutting edge increases, thus causing a problem that a binder phase existing in the cutting edge gradually reacts with a work material. Also, even if a hard coating is formed on the surface of a cermet comprising an exudation layer formed on the entire surface, the hard coating may peel off because of insufficient adhesion between the cermet and the hard coating.
On the other hand, in case of cutting a titanium (Ti) alloy used for aircraft industry, a cemented carbide tool comprising no hard coating formed thereon so as to prevent contamination of the worked surface is used. A Ti alloy has low thermal conductivity and high strength and is therefore known as a hard-to-cut material and, when a conventional cemented carbide tool is used, there arose a problem such as very rapid wear proceeding and short tool life.
Patent literature 3 describes that, when a sintered cemented carbide is subjected again to a heat treatment under a Co atmosphere to obtain a cutting tool made of a cemented carbide whose surface is coated with a very thin Co layer having a thickness of 8 μm or less and a Ti alloy is cut while spraying a coolant under high pressure using this cutting tool, tool life can be prolonged.
However, in the cemented carbide described in patent literature 3, although machinability of the Ti alloy is improved by the Co thin layer formed on the surface of the cemented carbide, if the temperature of the Co thin layer becomes higher during cutting, the Co thin layer may be welded to a work material. Therefore, the work material must be machined while spraying a coolant over the portion to be machined under high pressure, and thus there arises a problem that a large-scaled equipment for spraying a coolant under high pressure is required. Also, the Co thin layer is likely to be worn because of insufficient hardness, and thus there arises a problem that sufficient tool life is not obtained in case of machining at a high cutting speed.
Also, in case of cutting a Ni-based heat resistant alloy such as Inconel or Hastelloy, an iron (Fe)-based heat resistant alloy such as Incoloy, and a heat resistant alloy such as Co-based heat resistant alloy, a cutting tool comprising a cemented carbide and a hard coating formed on the surface of the cemented carbide is used. However, such a heat resistant alloy has high strength at high temperature, and thus there arises a problem that wear of the cutting tool proceeds at an initial stage.
On the other hand, various studies on an improvement in characteristics of the cemented carbide have been made and materials having higher hardness, higher toughness or higher strength have been developed according to the purposes. For example, patent literature 4 describes that, when a cemented carbide is produced by adjusting the content of a binder phase so as to controlling saturation magnetization to 1.62 μTm3/kg or less per 1 weight % of cobalt (Co) and a coercive force to 27.8 to 51.7 kA/m while suppressing segregation of a Co component, fractures in the cemented carbide decrease to impart high deflective strength, and thus a cutting tool suited for drilling or milling can be obtained.
Also, patent literature 5 describes that when using, as a cemented carbide used generally in the cutting field and wear resistant parts, a high toughness cemented carbide having a fine particle structure in which saturation magnetization per 1 weight % of cobalt (Co) is 1.44 to 1.74 μTm3/kg, a coercive force is 24 to 52 kA/m and a mean particle size of less than 1 μm, and the number of coarse WC particles (hard phase) having a particle size of 2 μm or more is only 5 or less, it becomes possible to achieve high toughness and to avoid sudden fracture event.
However, the cemented carbides having a coercive force of 24 kA/m or more disclosed in patent literature 4 and patent literature 5 is not suited for severe cutting such as cutting of a titanium (Ti) alloy or a heat resistant alloy because of too thin binder phase and too high hardness, and thus there arises a problem that sufficient fracture resistance cannot be obtained because of insufficient toughness of the cemented carbide.
Patent literature 6 describes that, by controlling a mean particle size of a cemented carbide within a range from 0.2 to 0.8 μm, saturation magnetization theoretical ratio within a range from 0.75 to 0.9, and a coercive force within a range from 200 to 340 Oe, the resulting cemented carbide has improved toughness and hardness and is best suited for use as a material of a precision die.
However, in the cemented carbide described in patent literature 6, since a hard phase has too small particle size, fracture resistance enough to be used for severe cutting of a Ti alloy or a heat resistant alloy cannot be obtained. Also, in the method disclosed in patent literature 6, since the cemented carbide is sintered by spark plasma sintering, there arises a problem such as low productivity and high cost.
Patent literature 7 describes that a cemented carbide comprising about 10.4 to about 12.7 weight % of a binder phase component and about 0.2 to about 1.2 weight % of Cr, which has a coercive force of about 120 to 240 Oe, saturation magnetization of about 143 to about 223 μTm3/kg of cobalt (Co) and a particle size of tungsten carbide (WC) particles (hard phase) of 1 to 6 μm, and is also excellent in toughness and strength and has high fracture resistance, and is useful as a cutting tool for milling a Ti alloy, a steel or a cast iron.
However, the cemented carbide described in patent literature 7 has high fracture resistance because of high content of the binder phase, but has not enough wear resistance to cut a Ti alloy or a heat resistant alloy. Also, when the content of the binder phase is too large, reactivity with a work material increases and a Ti alloy is likely to be welded to a cutting edge of a cutting tool, and thus there arises a problem such as deterioration of forming accuracy such as deterioration of quality of the worked surface, and tool damages such as chipping of cutting edge and abnormal wear.    Patent literature 1: Japanese Unexamined Patent Publication No. 2-221373    Patent literature 2: Japanese Unexamined Patent Publication No. 8-225877    Patent literature 3: Japanese Unexamined Patent Publication No. 2003-1505    Patent literature 4: Japanese Unexamined Patent Publication No. 2004-59946    Patent literature 5: Japanese Unexamined Patent Publication No. 2001-115229    Patent literature 6: Japanese Unexamined Patent Publication No. 1999-181540    Patent literature 7: Published Japanese Translation No. 2004-506525 of the PCT Application